With National Geographic’s recent announcement that it will begin production on a potentially record-breaking freediving event with diver William Trubridge, we thought we’d dig deeper into what it feels like to take the plunge.

We recruited Stepanek, a 13-time freediving record-setter, to walk us through what it’s like to dive hundreds of feet under water.

Stepanek can hold his breath for over 8 minutes and was the first person to dive over 400 feet on a single breath, a feat few freedivers have accomplished.

Perhaps the weirdest thing about freediving is that anyone can do it. “It is something that our body is designed for,” said Stepanek, the founder of Freediving Instructors International. Because we share a common lineage with aquatic mammals, our bodies still have some of their aquatic adaptations, he said.

While no one recommends taking up freediving without the proper instruction, the truth is that almost anyone with the proper training can learn to hold their breath for at least three minutes. When swimming downward at three feet per second, a person could make it to 100 feet and back in a little over a minute.

When this happens to people in Stepanek’s courses, they “realise they are capable of things they don’t believe possible. It almost redefines the world for them,” he said.

But don’t jump into the Hudson just yet. “The important thing is to know how to use the reflex and to know the dangers that are out there,” he said.

Still interested? Here’s a list of just a few of the strange sensations you might encounter while taking the otherworldly plunge.

Stepanek begins a freedive.

Source: YouTube/apneapatagonia

1. Heart slows

Since you get only one breath, you have a finite supply of oxygen for your dive. This means that the name of the game is oxygen conservation. Luckily, your body already knows how to do this.

When a human starts a freedive, just as when a whale does, the heart automatically begins to slow — a phenomenon known as the mammalian diving reflex. By slowing down the heart, the body conserves its precious oxygen.

Untrained divers may be able to lower their heartbeat by about 8% of their normal heart rate, said Stepanek.

Professional divers can summon this reflex in a few minutes and drop their heart rate by 50%, Stepanek said.

2. An elephant on your chest

“The first and most significant [sensation] is the experience of pressure, not only on the chest but on the eardrums and sinuses,” Stepanek said.

Thirty-three feet of seawater is the pressure equivalent of one atmosphere. This means that when you hit 33 feet your body is feeling twice the pressure it would at sea level. As a result, “your lungs will pretty much shrink to half of their size,” Stepanek said.

“With my dives going over 400 feet, you can imagine how much they compress,” said Stepanek, who likens it to an elephant stepping on your chest.

Professional freediver Niki Stepanek checking out a reef in Hawaii.

Source: Martin Stepanek

3. Urge to breathe… out

If you try holding your breath, at some point — maybe about 45 seconds to a minute — you will feel the urge to breathe. Despite a common misconception, it is not for want of oxygen. The air we breathe out still contains about 16% oxygen, compared to the 21% oxygen air we breathe in.

Other factors are at play in the desire to breathe, and our body has many triggers that set off this desire.

The first trigger is an excess of carbon dioxide in our body, a waste gas from our cells that’s expelled when we breathe out. “This urge to breathe works great on dry land, but does not work all that well under water because all that counts under water is the oxygen,” Stepanek said.

Elite freedivers have to train extensively to ignore this urge.

4. Urge to breathe… again

Stepanek hanging with the dolphins.

Source: Freediving Instructors International

This time, it is not because of carbon dioxide.

When your lungs get small, which happens when you exhale, your body gets the signal to breathe in. However, the pressure from the ocean also causes your lungs to shrink up — their spongy texture can’t stand up to the surrounding pressure.

At 66 feet under water, you are feeling three times the pressure and your lungs are a third the normal size.

“Your brain sees it as your lungs being exhaled,” Stepanek said, and tells your body to breathe in and inflate the lungs.

5. That Drunk Feeling

At extreme depths, the nitrogen in the air we breathe becomes inebriating. Jacques Cousteau famously called it “the rapture of the deep.” Scientifically called, “nitrogen narcosis,” this sensation may feel like the rapture of a few martinis.

As you dive deeper, the pressure on your body increases. This pressure pushes the nitrogen in your lungs into your bloodstream. The more pressure the diver is under, the more nitrogen gets absorbed by the body.

The nitrogen is especially well absorbed by fatty tissue, like the brain. When it gets into brain cells the nitrogen overwhelms them, impacting our thinking and ability to make decisions, essentially making us drunk.

6. Sinking

Stepanek sinks all the way down to the finish line. In this case, almost 273 feet.

Source: YouTube/flkitesurfer ricki

There is a point for every freediver at which they will no longer need to swim downward. They’ll just sink. The human body naturally floats in seawater. If you swim five feet down in the ocean, your body’s natural tendency is to float to the surface.

However, at a certain depth, the weight of the water on your body causes it to sink. The depth at which the sinking begins varies depending on many things — among them, a freediver’s swimming style.

On a 400-foot dive for Stepanek, this sinking begins at about 110 feet. “So then I have about 300 feet of just taking the ride,” he said.

7. Temporarily Color Blindness

Stepanek has trained himself to ignore the urge to breathe from carbon dioxide buildup or pressure, because neither gives him information on the most important factor: his oxygen level.

With every second that passes, the freediver uses up more precious oxygen. So how do they know when it’s really time to breathe? Some go color blind.

“The parts of your eyes that monitor colors are very demanding on oxygen. When the level of oxygen drops, even a little bit, you’re going to lose the color in your vision. So that could be one of the first signs,” Stepanek said.

“I first lose the color in my vision. And to me it’s like, OK, that’s it,” Stepanek said. Time to come up.

8. Loss of Peripheral Vision

But if he doesn’t, the loss of color vision can be joined by a loss of peripheral vision — the ability to see to the right and left of what’s in front of him.

“It’s almost like tunnel vision,” Stepanek said. “And that’s kind of when I would be pushing my luck.” When he can see only about 20 degrees of normal vision, Stepanek may start to see a flickering spot.

“Usually, that is a very bad sign. That would be followed by the tunnel closing completely and that would be unconsciousness,” he said.

9. Fingers And Toes Become Difficult To Move

When the body’s oxygen levels are dangerously low, hardly any oxygen gets to the fingers and toes. To conserve oxygen, the body has slowly been stealing it from the extremities and giving it to the vital organs. On the swim back to the surface, Stepanek will feel a burning sensation in his arms and legs for the same reason.

10. Surfacing Protocol

Obviously, it is important for divers to conserve enough oxygen to maintain consciousness and get themselves back to the surface. However, they also need some oxygen for what is perhaps the hardest task of the entire dive: the surfacing protocol. The inability to complete this protocol has led to the disqualification of more than one record-breaking dive.

The surfacing protocol is a series of tasks a competitive diver must complete within 15 seconds of reaching the surface. It consists of a series of three gestures, in order: taking off the mask, giving the “OK” signal with their hand, and saying, “I am OK.”

Those required tasks are how judges test if the divers are fully coherent, said Stepanek. They might seem like menial tasks, but someone dangerously low on oxygen will be in a state of confusion and can easily mess them up. To encourage safety, divers who cannot complete all these signals, in order, are disqualified.

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